Automotive finishes are applied in a series of coating layers, with each coating layer providing an important function in the performance of the composite finish. For instance, primer coating layers are used to protect the substrate from corrosion, chipping, and delamination of the coating from the substrate. Surfacer and primer surfacer layers are commonly used to provide a smooth surface upon which to apply the topcoat layers, and may add increased corrosion protection or chip protection. The topcoat layers provide beauty as well as protection against scratching, marring, and environmentally-induced degradation.
The topcoat coatings may be applied as a single, colored layer, but today most OEM automotive topcoats are applied in two coating layers—a colored basecoat layer and a transparent clearcoat layer. Automotive clearcoats have many performance requirements. They must be smooth and glossy to provide the desired aesthetic appeal. They must also be durable, both to preserve the coating appearance and to protect the steel substrate, by resisting scratching and marring and also degradation from UV light in sunlight, environmental etching, and heat.
Two of the most important functions of an automotive topcoat are resistance to weathering, including resistance to environmental etch, and resistance to scratch and mar. Clearcoat coatings lose both resistance to environmental etch and resistance to scratch and mar due to photooxidation of the coating surface during outdoor weathering. Ultraviolet light (UV) absorbers are used to prevent photooxidation, but these are incorporated throughout the coating, not just at the surface where environmental etch, scratch and mar occur. The UV absorbers are costly; it is expensive to use more UV absorber in the coating than is used or needed. Further, because they are not bonded to the coating layers, the UV absorbers tend to be lost from the coating surface, with consequent loss of protection from photooxidation.
Thus, it would be desirable to be able to reduce the amount of UV absorber used in the coating while at the same time using the UV absorber more effectively and with better retention in the coating during weathering.